The position of closest approach, i.e. the shortest distance between the Sun and the planet, is known as the perihelion (from the Greek peri = near and helios = Sun). At this point in the orbit, the planet is moving at its maximum speed (Kepler’s Second Law). The perihelion refers specifically to orbits aroundd the Sun, and is equivalent to the periapsis of a general orbit.
In a strong gravitational field, the location of perihelion may advance on successive orbits. Within the Solar System, this is most easily seen in the orbit of Mercury, providing an important test of General Relativity.